Great answer, thanks! Embarrasingly, I didn't even know about className :-)
Edit: please don’t take this as criticism. I love the project and think it’s awesome! That’s why I took the time to read through the source and grok it!
What properties are you talking about here? Despite the name, classes are really an unordered set, the order of classes on the element should not matter: when CSS gets applied, properties are prioritised based on the most specific rule, then the latest rule. That is the prioritisation of CSS properties should depend entirely on the CSS and not in any way on the order of the class attribute / className property.
Edit: Woah... It looks like I am incorrect. Nifty! I'm amazed I have never been burned by that before!
Edit 2: I suspect the reason I've never been burned by that before is because overrides like that have generally been applied over top of some kind of generic CSS file and the generic one was loaded first; since whichever rule is declared later is considered to have precedence, the override file wins over the generic file. My mind is kind of blown right now that over 20 years of web development I have never encountered this problem!